This invention relates to a process for shaping fiber reinforced thermoplastic articles in a molding or stamping operation. More particularly, it relates to shaping such an article wherein a preshaped preform of the final article is preheated in an oven, then transferred to a substantially unheated mold where the preform is compression-consolidated followed by tempering the molded article in an oven to provide predetermined structural integrity.
The rapid molding of thermoplastic resins with or without reinforcement into shaped articles is known in the art.
Nairn and Zoller discussed the effects of matrix crystallization in composites (JJA Nairn and P. Zoller, V International Conference on Composite Materials ICCMV and J Matl. Science, 1985 (20)). The analysis centered on the large dimensional changes involved in cooling semi-crystalline matrices from high temperature melts to room temperature and the associated strains imposed by shrinkage and constraint of reinforcing fibers. FIG. 1 is a plot of the magnitude of the shrinkage for several polymers derived from PVT (pressure-volume-temperature) measurement. Nairn measured the strain optically on an amorphous matrix resin where the retardation at the fiber matrix interface could be followed, and the results showed substantial stress build-up at the interfaces.
The conclusion was that thermoplastic materials, with a large temperature difference between forming temperature and room temperature, will show substantial internal strain. When crystallization of the matrix is superimposed in a heat-crystallize-cool cycle, resulting strain levels may be above those the matrix can tolerate and result in actual fraction of the sample. It is clearly important to minimize the strain effects on temperature cycling discussed by Zoller and Nairn.
The levels of strain which may be encountered in a semi-crystalline polymer on cooling from the melt to room temperature can be seen directly on a PVT curve where specific volume is plotted against temperature. The increase in specific volume (=1/density) measures the thermal expansion on heating a sample from room temperature into the melt. A cooling curve will normally follow a different path, but the melt and room temperature specific volumes will be similar to the heating values.
P. Aoller and P. Bolli, J. Macromol. Sci. Phys., B18, 555 (1980) disclose heating and cooling curves for polyethylene terephthalate (PET). Nylon 6,6, polypropylene (PP), polyether ketone ketone (PEKK), polyarylate (PAR), amorphous copolyamide (J-2) and an amorphous polyester (PETG). These data can be helpful in the task of minimizing strain related flaws in composites.
If crystallizable polymers are cooled slowly from the melt, they will in general crystallize. PE and PP will crystallize rapidly at only moderate supercooling below the normal melting points. PET will crystallize slowly at a higher (30.degree.-40.degree. C.) supercooling while a PAR such as DPPG-I may not crystallize in a reasonable time. If the polymers are cooled rapidly from the melt, the differences are more striking. PE and PP will crystallize. Polymers such as PET, PAR or PEKK can be obtained as an amorphous glass below their glass transition temperatures. These amorphous polymers are less dense than their crystalline counterparts and tend to have much higher elongations than the crystalline counterparts. On reheating above Tg, these amorphous materials can relax (and relieve stress) and crystallize.
In general, there is a second temperature at which strain relaxation can occur in crystalline polymers. This is the zone where the specific volume curve begins to increase before melting. Heating into this zone can relieve strain but in some polymers can lead to formation of large spherelites.
In usual forming operations of composites, a relatively standard cycle occurs. The system is heated to the melt, compressed to shape and then cooled, frequently at a leisurely pace. Since cooling is at the face, freezing/ crystallization occurs there first, while the bulk of the matrix resin is still liquid. As cooling continues, the crystallization front moves inward with concomittant shrinkage. This, in combination with the interfacial strain, imposes large stresses on the already crystallized material. For crystalline polymers with high elongation, such as PE and PP, the stress can be accommodated. For crystalline polymers which have low elongations to break, this can impose stresses leading to failure (flaw formation) or a state where relatively small additional strains can lead to failure. The effects can be quite significant in thick cross sections and show up as lowered strength and toughness.
The effects of a crystallization wave can be illustrated by following events in a thick rod of PET. When the molten polymer is formed, the rod diameter conforming to the mold will be made up of liquid with a specific volume .gtoreq.0.83 cc/g. As the surface layer crystallizes, its specific volume will decrease while that of the liquid core remains essentially unchanged. As the crystallization wave moves inward (heat flows out to the mold) the surface, already under compression from the mold and its own shrinkage, experiences further stress from shrinkage of the inner crystallizing layers. Some compressive failure occurs until the crystallizing cylinder becomes thick enough to support the compressive load. At this point, further crystallization occurs and is associated with a large negative pressure of the order of 200 MPa is we assume symmetry of the shrinkage and compressive forces.
Experimentally, these effects can be seen in two ways. Polishing a cut cross section shows cracks throughout the cross section. The largest occur off center in the high negative pressure zone and proceed to the surface at an angle to the glass fiber direction. If the polishing is done by hand, it will be observed that the outer layer--the surface layer--of the cylinder is soft and easily abraded. Proceeding inward, a harder section is seen followed by another soft section and another hard section at the core. However, the problems involved in the rapid molding of thick sectioned fiber reinforced resin parts, especially those with high loading of reinforcement (&gt;40% volume), for use in applications requiring high rigidity, structural integrity and geometric accuracy has not been addressed in the art.